The present invention relates to data communications networks and more particularly to a converged service for transporting frames between interconnected local area networks employing the same or different frame protocols.
Initially, computer systems conformed to what could be described as a host-centric model. Individual users interacted with a single, central host computer or mainframe through workstations attached to the mainframe either directly or indirectly through devices commonly referred to as cluster or display controllers. Virtually all of the data processing power resided in the mainframe with the individual workstations having relatively little data processing power or xe2x80x9cintelligencexe2x80x9d. The mainframe controlled all communications among workstations or between workstations and system resources, such as disk drives and printers. Assuming that the exchanges of data between workstations or between workstations and system resources could be characterized as xe2x80x9ccommunicationsxe2x80x9d, the communications were localized in nature.
Data communications networks evolved as a consequence of a need for users attached to different mainframes to communicate with each other. Early data communications networks consisted of a number of isolated host-centric systems which were connected through public or private switched telephone networks. Communication controller devices provided the interfaces between the mainframes and the switched telephone networks. In such data communications networks, the mainframe in each of the connected host-centric systems played a central role in management of the connections between workstation users in the different systems.
Over time and as a result of improvements in computer technology, the processing power originally confined to mainframes began to appear in smaller and smaller computers including devices now commonly referred to as personal computers. Over a relatively short period of time, personal computers were developed that possessed the processing power formerly found only in mainframes.
To take advantage of the distribution of processing power to personal computers and of improvements in data communication technology, new types of networks, generically referred to as local area networks or LANs, were developed. At the most basic level, a local area network or LAN consists of a plurality of workstations or other devices (such as printers or disk drives) which are all connected to a shared communications medium. Any device attached to a shared communications medium may be generically identified as a xe2x80x9cstationxe2x80x9d. Data to be exchanged between stations in the same local area network is written onto the communications medium by the source station and then copied from the medium by the destination station. Even where the area network is connected to a mainframe, the mainframe plays no significant role in the transfer of data between devices in the same local area network.
While several different kinds of local area networks have been proposed, the two most common kinds in current use are Ethernet and Token Ring local area networks. There are significant differences between these two kinds of local area networks.
An Ethernet local area network may also be referred to as a Carrier Sense Multiple Access/Collision Detect or CSMA/CD network. In an Ethernet local area network, all stations have an equal opportunity to transmit information across the shared bus. If a station has information to transmit, it first listens to the shared bus to determine whether any other station is already using the bus. If the bus is found to be idle, the listening station begins to transmit data. If the bus is found to be busy, the listening station waits for a predetermined period of time before trying to access the bus again. Information transmitted onto the bus is received by every station attached to the bus; that is, the information is broadcast. A station for which the information is intended recognizes its own address in the information and accepts the transmitted information. All other stations discard or ignore the information.
Token Ring local area networks use a logical ring to which all stations are connected. Any station who wishes to transfer information onto the ring must first acquire a xe2x80x9ctokenxe2x80x9d, which is a special data structure or frame. Once the station has the token, it may transfer information onto the ring. The information flows in one direction around the ring and through each of the connected stations. If a station is not intended to receive the information, it simply passes the information on to the next station on the ring. If a station is intended to receive the information, it copies the information into station memory for its use but still passes the information on to the next station on the ring. The information remains on the ring until it traverses the entire ring and reaches the source station. The source station strips the information from the ring and writes a xe2x80x9cfreexe2x80x9d token back on to the ring The token circulates around the ring until it is acquired by another station having information to transmit.
Both Ethernet and Token Ring technologies have their virtues. Ethernet installations tend to be less expensive than Token Ring installations. On the other hand, Token Ring networks can handle larger frame sizes than Ethernet networks, can accommodate a frame handling technique known as source routing and can accommodate different priority levels of data. These capabilities allow load balancing, automatic route recovery without session loss and use of redundant addresses in Token Ring local area networks.
Both Ethernet and Token Ring local area technologies are defined by specifications issued by the Institute of Electrical and Electronic Engineers (IEEE). Ethernet local area network technology is defined in IEEE 802.3 specifications while Token Ring local area network technology is defined IEEE 802.5 specifications. The IEEE specifications define many aspects of each of these local area network technologies, including the structure of the data frames which are used to transport information within the local area network. For reasons which are not important to an understanding of the present invention, the frame structures used in Ethernet local area networks are different than the frame structures used in Token Ring local area networks.
If individual local area networks were always to remain isolated one from another, the differences in frame structures would pose no problem. However, because users"" needs to communicate are not limited to the particular local area network to which they are attached, it is desirable to interconnect different local area networks to permit users on those different networks to communicate as if they were on the same network. One problem in interconnecting local area networks is that such networks may not be directly connected with another but may be separated by intervening networks, including wide area networks following a system architecture unlike any local area network architecture.
Techniques have been developed to interconnect local area networks even where those networks are linked only by an intervening network. One known technique requires that the intervening network encapsulate the local area network data into a frame or packet structure which the intervening network can handle. This may require that the local area network data be segmented at the point of entry into the intervening network and then reassembled at the point of exit from the intervening network. The encapsulation approach has the drawback that end-to-end transmission efficiency is degraded since the frame or packet structure used in the intervening network will add a considerable number of non-data bits to the information flow. Routing techniques have also been use to transport the data between separated local area networks. The routing techniques also degrade the transmission efficiency between the two end stations on the communications path.
Clearly, it is desirable to allow local area networks to be interconnected using a frame structure at least substantially similar to the frame structure employed in the local area networks themselves. Ethernet networks consisting of multiple, interconnected Ethernet local area networks have been developed. Similarly, Token Ring networks consisting of multiple, interconnected Token Ring local area networks have been developed. It is possible for an end station in a given Ethernet local area network to communicate with another end station in a remote Ethernet local area network through such Ethernet networks using a frame structure substantially similar to the,standard Ethernet frame structure. Similarly, it is possible for an end station on a given Token Ring local area network to communicate with an end station on a remote Token Ring local area network using a frame structure similar to that employed within a Token Ring local area network. Frames used in intervening networks which connect Ethernet-to-Ethernet or Token Ring-to-Token Ring local area networks may differ from frame structures used within the local area networks due to the need to include information required to address the remote local area network.
If users connected to a particular type of local area network (either Ethernet or Token Ring) only wish to communicate with users attached in the same type of local area network, the known approaches described above would probably satisfy their needs. There are, however, many Ethernet and many Token Ring local area networks also in use today. Clearly, situations will arise where an Ethernet user will wish to communicate with a Token Ring user and vice versa.
The present invention is a converged service which permits users of either an Ethernet or a Token Ring local area network to communicate with other local area network users regardless of whether those users are connected to the same or the opposite type of local area network.
The invention is implemented in converged service edge devices located at the boundaries between traditional local area networks and an intervening network. An edge device accepts an incoming data frame and determines whether it originates at a Token Ring station. If the frame does not originate at a Token Ring station, by definition it must originate at an Ethernet station. Any frame originating at an Ethernet station is modified by adding a virtual LAN tag field containing information which will allow the frame to reach its ultimate destination across an intervening network. The frame is also modified by adding a length field defining the length of the frame. An ordinary Ethernet frame has a maximum length of 1500 bytes. Token ring frames are modified by deleting selected fields and then adding the virtual LAN tag. Regardless of the type of the frame, the included frame check sequence character must be recalculated to reflect the noted changes in the frame contents.